Process of coating metal castings



United States Patent 3,450,189 PROCESS OF COATING METAL CASTINGS Donald Francis MacDonald, Snffern, N.Y., assignor to The International Nickel Company, Inc., New York, N.Y., a corporation of Delaware No Drawing. Filed Aug. 22, 1966, Ser. No. 573,782 Int. Cl. C23c 1/10; B22d 23/00 US. Cl. 164-95 8 Claims ABSTRACT OF THE DISCLOSURE This invention relates to protective and decorative coatings and to methods of applying these coatings to metal castings.

Many cast materials such, for example, as cast iron find numerous applications because of their relatively low cost and good engineering properties. However, ordinary cast irons have poor corrosion resistance and, where the iron must withstand corrosive environments, a more expensive grade of alloyed cast iron having superior corrosion resistance is generally necessary. For example, valves used in a system which handles corrosive solutions are typically made from alloy cast iron containing nickel in amounts up to 35%, since corrosion resistance is of primary importance for this particular application. Although only part of the valve surface is usually in contact with the corrosive environment, the entire valve is generally made from the more expensive grade of cast iron. It has been proposed, therefore, that the entire casting need not be made from the more expensive, corrosion resistant material and, instead, substantial savings can be achieved if the casting is made entirely from a lower grade, less expensive material so long as those surfaces exposed to the corrosive environment are coated with a corrosion resistant material such, for example, as nickel. The casting may be nickel coated, according to the suggestion, by immersing the casting in an electrolytic bath and electroplating nickel only on the surfaces which will be exposed to the corrosive environment. Such a solution to the problem is not without its shortcomings. Electroplating nickel on a casting is an expensive and time consuming process especially when the castings are to be mass produced. Another objection to this method of coating exposed surfaces is that all areas of the body which are immersed in the electrolyte, instead of just those areas of the surface for which the corrosion resistant coating is primarily intended, will be electroplated (coated) unless covered or masked with an inert material. These materials are usually applied with a brush, allowed to dry, and thereafter removed when electroplating is completed. Consequently, this process requires considerable time and storage space, and involves a costly materials handling problem for the foundry.

Other known methods of applying protective or decorative coatings to metal castings involve welding a plate or covering to the casting or inserting plates or discs into the mold and then pouring molten metal into the mold and in contact with the insert. The insert plate or disc has been found, with varying degrees of success, to adhere to the casting. Both of these methods, however, are limited to covering only simple shaped surfaces of a Patented June 17, 1969 casting. Great difiiculty has been encountered in covering intricate shapes or thin sections.

Although many attempts were made to overcome the foregoing difliculties and other difliculties, none, as far as I am aware, was entirely successful when carried into practice commercially on an industrial scale.

I have now discovered that protective or decorative coatings of nickel and other materials may be formed on preselected surfaces of a met-a1 casting during the actual casting process thus eliminating time consuming and expensive operations; The quality of these new coatings compares favorably with those formed by electroplating and by other known methods.

It is an object of the present invention to provide a method of coating metal castings for protection from corrosion or erosion or for decorative purposes.

Another object of the invention is to provide surface alloying of a metal casting which increases the surface hardness of the casting.

The invention also contemplates providing a coating on a metal casting which gives the casting 'a decorative, or distinctive appearance.

It is a further object of the invention to provide a coating upon selected surfaces of a sand casting of any size or of the most intricate shape.

Generally speaking, the present invention contemplates a process or method of coating metal castings which comprises: preparing a slurry containing a powdered metallic coating material, a binder, and a liquid diluent or carrier; applying the slurry to a preselected area of a surface which determines or defines at least part of the desired shape of the casting, i.e. the surface of a sand or other ceramic core, a mold wall or a consumable pattern or combination thereof, so as to form a coating on the preselected area; drying the coating; assembling the mold; and, thereafter, pouring molten metal into the mold and in contact with the coated surfaces whereby upon solidification a casting is formed having an adherent metallic coating. The coating adheres to and becomes part of the surface of the casting. A protective or decorative coating is thus formed on preselected surfaces of the casting. As indicated hereinbefore, the slurry may be applied to the core and/ or to some other surface of the mold cavity in which the molten metal is poured. The slurry may also be applied to a consumable pattern, e.g., a polystyrene pattern, which temporarily occupies the space in which the molten metal is to be poured and which is volatilized out of the mold by the heat of the molten metal or otherwise. When the slurry is applied to the consumable pattern, it is then dried and placed in the mold with the dried coating in contact with a portion of the mold cavity surface and in effect becomes a coating on the surface when the pattern is volatilized.

In carrying the invention into practice, the powdered metallic coating material may be an elemental metal powder or alloy or a combination of powdered metals and/ or alloys, such, for example, as nickel powder, chromium powder, a mixture of copper and nickel powders, stainless steel powder, nickel-copper alloy powder and powders of other nickel-containing alloys, to form protective or decorative coatings on metal castings. Moreover, it is unnecessary for the metallic casting material to have a melting point below the coating material. Where, however, the casting material, for example, cast iron or steel, melts at a temperature substantially in excess of the coating material, only thin sections of the casting should be coated to assure that heat transfer sufficient to cause excessive melting of the coating will not take place.

The mold is advantageously made from green sand because the evolution of gas from the green sand mold after contact with the molten metal results in a reducing atmosphere over the coating which prevents, or at least minimizes, oxidation of the coating. However, dry sand molds can also be employed although they are not as preferred as green sand molds. The coating material can be applied to the surface against which the poured metal solidifies, e.g., the core or mold, or both, in the form of a slurry or paste by painting, spraying or dipping. In preparing the slurry, it has been found advantageous to mix the coating material, which is in the form of a powder, with a diluent or carrier such, for example,.as water. Although particle sizes up to about minus 100 mesh can be used, I have found that finer particle sizes, e.g., minus 200 mesh in size, remain suspended for much longer periods and for this reason are easier to work with. Although water is by far the most practical diluent or carrier for the coating material from the standpoint of convenience and safety, other liquids which are not flammable at elevated temperatures may conveniently be used.

The slurry must also contain a binder which causes the particles of coating material to adhere to one another and to the core, mold or consumable pattern upon drying. The binder should have a low ash content and be amenable to use at high temperatures. While suitable binders in clude methylcellulose and sodium silicate, it is preferred to use sodium polyacrylate since it is more stable at higher temperatures. The binder should be present in the slurry in amounts of at least about 0.05%, or the slurrycoated core or mold will not be sufficiently adhesive to withstand handling and, when the binder decomposes during contact with the molten metal, as is the case with methylcellulose and sodium polyacrylate, gas will not be emitted in amounts sufficient to substantially contribute to the reducing atmosphere which minimizes oxidation of the coating. If the binder is methylcellulose it should not be present in amounts substantially above 1% since the slurry becomes thick and unworkable and gas emission from the decomposed binder becomes excessive causing pinholes in the coating and in the casting itself. When, however, the binder is sodium silicate or sodium polyacrylate, amounts of about 0.05% to 35% of either may be used in combination with the aforementioned amounts of coating powder and carrier. While good results have been obtained when the mixture comprises by weight: about 65% coating powder, about 25% carrier and about sodium polyacrylate or about 75% coating powder, about 24.75% carrier and about 0.25% methylcellulose binder, it has also been found that satisfactory results may be obtained when, in combination with the hinder, the coating powder is present in amounts by weight of about 50% to 85%, and the balance is the carrier, usually in amounts of about to 50%.

The slurry or paste should be applied to the mold lining or core in amounts varying between and /2 inch in thickness. If the core or mold coating is less than about & inch thick, the coating may melt and dissolve in the molten metal and fail to form a continuous coating on the casting. Moreover, a thin coating may be washed away on the impact of the molten metal. On the other hand, if the coating on the core or wall is more than about /2 inch thick, the coating may tend to crack and peel off the core or mold wall. This tendency to crack will depend to some extent on the particular binder employed. The slurry can be applied to the core or mold by dipping or with the aid of a brush or a spray and, in the latter case, the viscosity of the slurry should be somewhat lower. I prefer to apply the slurry to the core or mold wall with a brush although satisfactory results may also be achieved by spraying or dipping. The core or mold on which the slurry has been applied is then allowed to dry in air at room temperature or, where only the sand core has been coated, in an oven, for example, at temperatures up to about 300 F. for about one hour.

After drying the coated core or mold, further applications of slurry can be made in order to increase the coating thickness followed by redrying. The mold is then assembled. Risers and gates should be so located in the mold as to minimize turbulence in the cast molten metal. The molten metal is then poured into the mold and in contact with the coated core or mold or both. As the molten metal freezes, the coating on the core or mold becomes bonded to the metal with which it is in physical contact and forms a coating on the solid metal casting. It has generally been found with nickel powder coatings that a slurry or paste coating of about 4! inch on the core or mold wall results in about a inch thick final coating on the casting although this will vary with the materials, shape of the coating, and pouring temperature. The pouring temperature of the molten metal becomes somewhat of a compromise. High pouring temperatures are necessary to assure adequate flow of metal in the mold while lower pouring temperatures reduce the chances of the molten metal washing away the coating (wash out), or a part thereof, contained on the core or mold. The pouring temperature for iron base materials should be maintained between about 2350 F. and 2950 F. Pouring temperature determines to a marked extent the type bond which exists between the metallic coating and the metal casting. The higher the pouring temperatures in the range indicated, the more readily an intermediate alloy layer forms which is composed of the cast iron-base material and the coating material, located therebetween, which serves to bond the materials together. This alloy layer results in an extremely adherent bond between the metallic coating and the metal casting. Lower casting temperatures than indicated herein result generally in inadequate flowability of the iron-base casting material. Advantageously, the pouring temperature is between 2600 F. and 2850 F.

For the purpose of giving those skilled in the art a better understanding of the invention, the following illustrative examples are given:

Example 1 A consumable pattern formed of polystyrene was prepared for use in the casting of a deck cleat for a boat.

Except for the base of the cleat which was used for gating purposes, the pattern was completely painted with a slurry of carbonyl nickel type powder having a minus 200 mesh particle size. The slurry had a composition in weight percent as follows:

Percent Type 100 nickel powder (coating material) 65 Water (carrier) 25 Sodium polyacrylate (binder) 10 Example 2 A consumable pattern was prepared and coated in the same manner as in Example 1. The slurry contained a powdered nickel base alloy having a minus 200 mesh particle size and containing about 28.9% copper, 0.34% iron, 0.91% silicon, 0. 04% carbon, 0.88% manganese, 0.001% phosphorous, 0.004% sulfur, and the balance essentially nickel. The slurry had a composition in weight percent as follows:

Percent Nickel base alloy powder (coating material) 65 Water (carrier) 25 Sodium polyacrylate (binders) 10 Type 1030 steel was then poured at 2900 F. into a sand mold containing the coated consumable pattern. The resulting casting was then sectioned and etched with 2% nital solution. An adherent and substantially continuous nickel-copper alloy coating appeared on the surface of the casting.

Example 3 Dry sand cores were made for a 1 inch gate valve mold and were prepared in the usual manner as follows: 100 pounds of silica sand, 1 pound of bentonite, and 1 pound of mogul binder were mixed together and mulled for 5 minutes. To this mixture was added, 2 quarts of linseed oil and 3 quarts of water and the resulting mixture molded into cores and then baked at 400 F. for 3 hours. The cores were thereafter coated with mica core wash and baked again at 400 F. for 3 hours. The dry sand cores were then painted with a slurry of carbonyl nickel type 100 powder having a minus 200 mesh particle size. The slurry had a composition in weight percent as follows:

Percent Type 100 nickel powder (coating material) 75 Water (carrier) 24.75 Methylcellulose (binder) 0.25

The coated cores were given a total of six separate coats which resulted in a final coating thickness of about /8 inch. The cores were dried and then placed in green sand molds which were designed for a valve body and the molds were thereafter fully assembled. Gray iron was poured at 2650 F. into a mold containing a coated core and also at 2750 F. into another mold containing a coated core. A nickel coating was formed on the surfaces of the metal castings which were in contact with the coated cores. The coated castings were then sectioned, polished and exposed in a furnace at about 500 F. for about one hour to oxidation. The nickel coating was resistant to oxidation whereas the gray iron of the body was oxidized. The cast valve bodies containing the nickel coating were sectioned, polished and examined under the metallographic microscope. A continuous and adherent nickel coating appeared on the surfaces of the metal castings which were in contact with the nickel-coated cores poured at both 2650 F. and at 2750 F. In addition, an alloy gradient of austenite and martensite appeared at the interface between the nickel coating and the gray iron of the cast valve body. Microhardness readings taken across the interface confirmed the fact that local alloying had taken place.

Example 4 A number of dry sand cores, prepared as set forth in Example 3, were painted with a slurry of carbonyl nickel type 100 powder having a minus 200 mesh particle size in the same manner as in Example 3. The coated cores were again given a total of six separate coats which resulted in a final coating thickness of about A; inch. The coated cores were then dried and placed in green sand molds which were designed for a valve body, and the molds were thereafter fully assembled. An austenitic alloy cast iron containing about 15.5% nickel, 2.8% carbon, 1.25% manganese, 2.2% silicon, 2% chromium, 6.5% copper and the balance essentially iron was poured at 2650 F. into a mold containing a coated core and also at 2750 F. into another mold containing a coated core. A nickel coating was formed on the surfaces of the metal castings which were in contact with the coated cores. The coated castings were then sectioned, polished and exposed to oxidation in a furnace at about 500 F. for about one hour. The nickel coating was more resistant to oxidation than the uncoated surfaces. The cast valve bodies containing the nickel coating which were poured at 2650 F. and at 2750 F. were sectioned, polished and examined under the metallographic microscope. A continuous and adherent nickel coating appeared on the surfaces of the metal castings which were in contact with the coated cores and which were poured at 2650 F. and at 2750 F. In addition, an allo gradient appeared between the nickel coating and the alloy cast iron of the casting.

Example 5 A number of dry sand cores, prepared as set forth in Example 3, were painted with the same slurry of carbonyl nickel type powder having a minus 200 mesh particle size as in Example 3. The cores were given a total of six separate coats which resulted in a final coating thickness of about /8 inch. The cores were then dried and placed in green sand molds which were designed for a valve body and the molds were thereafter fully assembled. An austenitic ductile iron containing 20% nickel, 2.8% carbon, 1.0% manganese, 2.2% silicon, 2% chromium, about 0.05% magnesium and the balance essentially iron was poured at 2600 F. into a mold containing a coated core, and also at 2650 F. into another mold with a coated core. A nickel coating was formed on the surfaces of the metal castings which were in contact with the coated cores. The coated castings were then sectioned, polished and exposed to oxidation in a furnace at about 500 F. for about one hour. The nickel coating was again found to be more resistant to oxidation than the uncoated surfaces. Pieces of the cast valve body containing the nickel coating which were poured at 2600 F. and at 2650 F. were sectioned, polished and examined under the metallographic microscope. A continuous and adherent nickel coating appeared on the surfaces of the valves poured in contact with the nickel coated cores both at 2600 F. and at 2650 F. In addition, an alloy gradient appeared between the nickel coating and the austenitic ductile iron of the casting.

Example 6 A dry sand core was prepared and slurry coated with carbonyl nickel as set forth in Example 3. The core was given a total of six separate coats which resulted in a final coating thickness of about A; inch. The core was then dried and placed in a green sand mold which was designed for a valve body and the mold was thereafter fully assembled. Ductile iron containing about 0.6% nickel, 3.5% carbon, 0.5% manganese, 2.4% silicon, about 0.05% magnesium, and the balance essentially iron was poured at 2600 E. into the mold containing the coated core and into a mold with an uncoated core. A nickel coating was formed on the surfaces of the ductile iron casting which were in contact with the coated core. The coated casting was then sectioned, polished and exposed to oxidation in a furnace at about 500 F. for about one hour. The nickel coating was more resistant to oxidation. The cast valve body containing the nickel coating was sectioned, polished and examined under the metallographic microscope. A continuous and adherent nickel coating appeared on the surfaces of the metal casting which were in contact with the coated core. In addition, an alloy gradient appeared between the nickel coatmg and the ductile iron of the casting. Sections of the nickel-coated valve body and the completely uncoated valve body were immersed in 5% by weight sulfuric acid water solution for about 24 hours at room temperature. The uncoated, sectioned surfaces of the coated sample were covered with sealer to protect them from attack by the acid while the coated surface of the same sample was left unprotected. The sectioned surfaces of the completely uncoated sample were partially covered with sealer to leave exposed an uncovered portion corresponding to, and of the same area as, the exposed nickelcoated surface. The uncoated sample was also exposed for 24 hours to the acid solution without further protection. The nickel-coated sample showed a substantially greater resistance to attack by the acid. The corrosion rate for the nickel coated sample was 362 mdd compared to 14,000 mdd for the uncoated sample.

Example 7 A dry sand core, prepared as set forth in Example 3, was painted with a slurry containing a powdered nickel base alloy having a minus 200 mesh particle size and containing about 28.9% copper, 0.34% iron, 0.91% silicon, 0.04% carbon, 0.88% manganese, 0.001% phosphorous, 0.004% sulfur and the balance essentially nickel. The slurry had a composition in weight percent at follows.

the sea water without further protection. Each of the nickel-coated samples showed a substantially superior resistance to attack by sea water. The results of a visual comparison in corrosion resistance between the nickelcoated and the uncoated samples are set forth in the table.

TABLE Corrosion Pouring resistance in temp. circulating Sample Casting material Surface F.) sea water Gray iron (Example 3) Nickel-coated (Example 3) 2, 650 Very good. o 2, 750 Fair. ...do.... Uncoated 2,650 Poor. Alloy cast iron (Example 4) Nickel-coated (Example 4)... 2, 750 Fair. Alloy ductile iron (Example 5).... Nickel-coated (Example 5)... 2, 650 Very good. 6.- o Unco e 2, 650 Fair. 7 Ductile iron (Example 6) Nickel-coated (Example 6) 2,600 Do.

Percent The present invention is particularly applicable to cast- Nickel base alloy powder (coating material) 75 Water (carrier) 24.75 Methylcellulose (binder) 0.25

The coated core was given a total of six separate coats which resulted in a final coating thickness of about /8 inch. The core was then dried and placed in a green sand mold which was designed fora valve body and the mold was thereafter fully assembled. Gray iron was poured at 2650 F. into the mold containing the coated core. A nickel-copper alloy coating formed on the surfaces of the metal casting which were in contact with the coated core. The coated casting was then sectioned, polished and exposed to oxidation in a furnace at about 500 F. for about one hour. The alloy coating was resistant to oxidation whereas the uncoated surfaces were oxidized. Pieces of the cast valve body containing the coating were sectioned, polished and examined under the metallographic microscope. A continuous and adherent nickelcopper alloy coating appeared on the surfaces of the metal casting which were in contact with the coated core. In addition, the alloy gradient appeared between the coating and the gray iron of the casting.

Example 8 A dry sand core, prepared as set forth in Example 3, was painted with the same slurry of nickel base alloy powder as in Example 7. The coated core was given a total of six separate coats which resulted in a final coating thickness of about A; inch. The core was then dried and placed in a green sand mold which was designed for a valve body and the mold was thereafter fully assembled. Type 1010 steel was poured at 2850 F. into the mold containing the coated core. A nickel-copper alloy coating formed on the surfaces of the metal casting which were in contact with the coated core. The coated casting was then sectioned, polished and exposed in a furnace at about 500 F. for about one hour in order to oxidize the sectioned surfaces. The nickel-copper coating was resistant to oxidation whereas the steel was oxidized. The cast valve body containing the coating was sectioned, polished and examined under the metallographic microscope. A continuous and adherent nickel-copper alloy coating appeared on the surfaces of the metal casting which were in contact with the coated core. In addition, an alloy gradient appeared between the coating and the 1010 steel of the casting.

Example 9 Sections of some coated valve bodies from Examples 3 to 6, and of some similar uncoated valve bodies of the same casting material as used in Examples 3 and 5, were exposed at room temperature for 72 hours to synthetically prepared, circulating sea water. The uncoated, sectioned surfaces of the nickel-coated samples were covered with sealer to protect them from attack by the sea water while the nickel-coated surfaces were left unprotected. The sectioned surfaces of the uncoated samples were partially covered with sealer leaving exposed a portion corresponding to the exposed, sectioned surfaces of the nickel-coated samples. The uncoated samples were then also exposed to ings exposed to erosive or corrosive environments such, for example, as couplings, elbows, or Ts exposed to erosive conditions in use, valves exposed to corrosive solutions, and castings employed in marine engines which are exposed to sea water.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

I claim:

1. A process of coating metal castings which comprises:

(a) preparing a slurry containing a powdered metallic coating material, a binder and a liquid carrier, wherein the slurry by weight comprises about 50% to of at least one powdered metallic coating material selected from the group consisting of nickel and nickel-containing alloys, a binder selected from the group consisting of about 0.05% to 1% methylcellulose, about 0.05% to 35% sodium silicate and about 0.05% to 35% sodium polyacrylate, and the balance essentially water as the liquid carrier;

(b) applying the slurry to a preselected area of a surface which defines at least part of the desired shape of the casting so as to form a coating on the preselected area;

(c) drying the coating;

((1) assembling a mold containing the coated area;

(e) and, thereafter, pouring molten metal into the mold and in contact with the coated area whereby upon solidification a casting is formed having an adherent metallic coating.

2. A process in accordance with claim 1 in which the binder is sodium polyacrylate.

3. A process in accordance with claim 1 wherein the powdered metallic coating material is carbonyl nickel powder.

4. A process in accordance with claim 1 wherein the molten metal upon solidification is a material selected from the group consisting of cast iron, ductile iron and steel.

5. A process in accordance with claim 4 wherein the temperature at which the molten metal is poured into the mold is between about 2350 F. and 2950 F.

6. A process in accordance with claim 1 wherein the coating material is in the form of a powder having a particle size capable of passing a 200 mesh screen.

7. A process in accordance with claim 1 wherein the slurry is applied to a core surface to form a coating thereon having a thickness between about inch and /2 inch.

8. A process in accordance with claim 1 wherein the slurry is applied to a consumable pattern to form a coating thereon.

(References on following page) References Cited UNITED 3,275,460 9/1966 Jeanneret 1175.2X

STATES PATENTS J. SPENCER OVERHOLSER, Primary Examiner.

Jacobs 164-75 X EUGENE MAR, Assistant Examiner. Williams et a1 16475 X 5 Toulmin 117 5.1 X Feagin 164-57 X 117-5.2; 16433, 58, 75, 138; 249-114 

